Process for production of chromic oxide



Nov. 6, 1951 J. SCHULEIN 4,

PROCESS FOR PRODUCTION OF CHROMIC OXIDE Filed Sept. 11, 1946 ORE l0 L/ME l w ROAST r A /-/20 r"" k 600A ASH I, V r 1 l LEACH r i l RES/DUE x 5 00 TREAT L I /I2 T TREAT 2 3 ALL/M r. --AC/0 T i r l l @0514. REEL/6E AGENT I \H.

I /5 BURN CHROM/C INVENTOR Patented Nov. 6, 1951 srArEs PATENT PROCESS FOR PRODUCTION OF CHROMIC OXIDE .Ios'eph Schiilcin, ccrvaniaoreg".

Application September 11, 1946, Serial N6.696,18i

3' claims. (01. 23-145 This invention relatesto: a processforproducing. chromic v oxide, CrzOs, and isespecially adaptedtoproduction of said compoundwhen starting from the ore'chromite, thoughtheinvention mayalso be used whenstartingatapoint furtheralong in the process how sheet as will be seen; as this specification proceeds.

In the past chromic oxide has been. produced by: a variety of' methods most of which call for reduction of a-.-hexavalen-t chromium-compound to. the. tri-valent state Many; such: processes introduce-sulfur or-compounds of sulfur into the product. One of the objects of the present invention is to eliminate the introduction of sulfur orsulfur compounds in the; finished chromic oxide.

Other processes requirethe use of a large amount of reducing-.agent as it is necessary to reduce all the chromium to the tri-valent state. In the present invention I have aimed at a method-of producing chromic oxide without the necessity ofreducing allo'f the chromium from the hexavalent to the trivalent state by the use of reducing agents; Other objects and advantages of my process will be seen as the specifica tionqdevelops While the roasting of chromite" ore igrold in the art and I do not claim, such a step as a part of my invention; I prefer" to illustrate my process starting with the raw process-at lows-recovery of sodaashwhich c'a" be re -usd in the roas'ting process; thusproducing a saving. To illustrate further I now refer to the accompanying flow sheet in which It represents a roasting operation in which the ground chromite is roasted with soda ash and other ingredients in a manner Well known to the art, I I represents the leaching operation, also well known; from the leach I prefer to run to a treating tank I2, wherein the leach liquor is treated with CO2 so as to adjust the pH to a point whereat alum, etc. is precipitated. This step is known to the art.

In this treat step soda ash may be recovered, if desired, for return to the roast. I now take the treated alkaline liquor containing sodium chromate to an electrolytic cell represented by 13, preferably a diaphragm type cell though this type is not necessary, wherein, by the passage of a direct current there is formed an anolyte liquor high in dichromate or chromic acid, and a catholyte liquor high in sodium hydroxide and. containing sodium chromate, this catholyte liquor can be retreated with CO2 if desired to form soda ash containing sodium chromate, which mixture can be returned to the roast for re-use. The

anolyte liquor need not be free of sodium= salts entirely,- and I preferto carryout the electrolysis until the anolyte consists of approximately'90-% chromic acid and 10% sodium dichromate in a total concentrationof approximately 200' grams of CrOa per liter. I now take this-anolyte liquor to a reducing tank M where wood flour is added to the solution either cold or at elevated: temperature. I have discovered that the woodflour is an excellent and cheap reducing'agent for this liquor. I prefer to use Redwood-flour though other wood flour alsogives good results? My object in this step is to reduce'only' a part of the hexavalent chromium to the tri-valen't state so that the tri-valentchromium' formed will react with the hexavalent chromium. inthe liquor to form chromium dichromate, C1'2(C12O7') a, which compound becomes only slightly soluble at pH s, above about 4.0'and thereforeprecipitatesout a a dark brown compound; I therefore'anal'yie my anolyte liquor and adjust theamount ofwood flour to be added accordingly, after first running tests on any particular wood; to determine: its reducing. power since-this varies from 'wood to wood. It is seen thatas the chromium dichro mate is formed and as it is only slightly dissociated, the pI-I'of the solution rises; finally-reaching avalue at which precipitationoccursjand itis a matter of chemical control as libz'the exact amount of wood flou-r ne'ededfor a given anolyte composition.

I prefer to add-some excess wood flour overthattheoretically needed to'produce the chromium dichromate since I have found that the excess wood flour is a decided help if a filtering step is undertaken as follows:

I may now filter the chromium dichromate precipitate and then burn said precipitate in a furnace l5, which may be of any type, I have used a rotary tube furnace very successfully for such burning, and under the influence of the burning, with air available, the chromium dichromate breaks down to the desired productgreen chromic oxide.

Instead of filtering as described above, I may add enough wood flour, which reacts very slowly with the anolyte liquor at pI-Is above about 4.5, to absorb most or all of the anolyte liquid remaining after the formation of the chromium dichromate precipitate, and burn the whole mass either directly or after drying to remove excess Water. In this case the excess wood flour burns off during the furnacing operation and therefore does not contaminate the final product.

It will be seen that the reduction step depends on the reaction between the wood flour and the dichromate, therefore the process could be operated by adding an inorganic nonreactive acid to the treat liquor so as to convert .the chromate to dichromate and lower the pH to a low enough value to permit the reduction reaction to take place as illustrated alternatively by dash lines at M, then, after reduction, the pH could be raised by the use of neutralizing chemicals, or electrolysis, to precipitate the chromium dichromate. I have found sulfuric acid suitable for this purpose. Obviously other reducing agents could be substituted for the wood flour, such as sugar, etc., but I prefer wood flour due to its low cost and ease of burning to remove excess. My use of chromium dichromate is the soul of my invention since this compound may be formed with only a part of the total chromium in the reduced state, thus materially decreasing the amount of reducing agent needed and providing a great saving over the prior art.

One variation of my process might be to add an inorganic acid to the treat liquor to reduce the pH to about 1.0 and convert the chromate to dichromate, then reduce this acidified dichromate liquor completely with a reducing agent such as wood flour, in this case, if sulfuric acid were the acid used, a solution of chromic sulfate and sodium sulfate would be formed, this solutlon could then be added to some unreduced treat liquor containing sodium chromate and thus the chromium dichromate would be precipitated with the formation of sodium sulfate in solution which can be filtered away from the chromium dichromate, or can be left in the mass and leached out of the chromic oxide after burning.

The principal reaction that I rely on to make my process successful is the reaction forming chromium dichromate from tri-valent and hexavalent chromium compounds.

I am aware that due to the equilibrium between the dichromate and chromate ions, that there will be some variation in the actual compound formed, varying from chromium dichromate to chromium chromate, and it is to be understood that my references herein to chromium dichromate means whatever mixture of chromium dichromate and chromium chromate that happens to be formed, this of course, varies with the dichromate-chromate ion ratio.

It will be seen that I have provided a new and novel method of producing chromic oxide, and while I am aware that numerous changes might be made in my flow sheet I have shown only my preferred form. Therefore, the above described methods should be construed as illustrative only and not as limiting the scope of the following claims.

I claim:

1. A method of producing chromic oxide which includes electrolyzing an aqueous solution of a chromate salt of an alkali metal so as to produce an anolyte solution consisting substantially of chromic acid, partially reducing said anolyte solution to form chromium dichromate thereby raising the pH of said solution to a value above about pH 4.5 to precipitate said chromium di.

chromate, separating said precipitate from said solution and thermally decomposing said chromium dichromate to produce chromic oxide.

2. A method of producing chromic oxide which includes electrolyzing an aqueous solution of sodium chromate so as to produce an anolyte solution consisting substantially of chromic acid, partially reducing said anolyte solution to form chromium dichromate thereby raising the pH of said solution to a value above pH 4.5 to precipitate said chromium dichromate, separating said REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,099,658 Pearson et al Nov. 16, 1937 2,416,551 Udy Feb. 25, 1947 OTHER REFERENCES Mellor,Inorganic and Theoretical Chemistry, vol. 11, pages 206, 20'! and 343. Published by Longmans, Green and Co., London (1931). 

1. A METHOD OF PRODUCING CHROMIC OXIDE WHICH INCLUDES ELECTROLYZING AN AQUEOUS SOLUTION OF A CHROMATE SALT OF AN ALKALI METAL SO AS TO PRODUCE AN ANOLYTE SOLUTION CONSISTING SUBSTANTIALLY OF CHROMIC ACID, PARTIALLY REDUCING SAID ANOLYTE SOLUTION TO FORM CHROMIUM DICHROMATE THEREBY RAISING THE PH OF SAID SOLUTION TO A VALUE ABOVE ABOUT PH 4.5 TO PRECIPITATE SAID CHROMIUM DICHROMATE, SEPARATING SAID PRECIPITATE FROM SAID SOLUTION AND THERMALLY DECOMPOSING SAID CHROMIUM DICHROMATE TO PRODUCE CHROMIC OXIDE. 